WHAT GOES WRONG?
Timing belts rarely snap. More commonly, the fibre teeth will shred, or else the belt simply falls from its pulleys. Naturally, leaving the belt past a reasonable replacement interval can lead to excessive deterioration of its structure. Yet, as a timing belt tends to not only drive the camshaft but also other items, such as the oil/ water/high-pressure fuel pumps, or the balance shafts, any wear of those items, including within their bearings, will place the belt under additional stress. The belt is likely to be routed around idler and tensioner pulleys, with deteriorating bearings on those components affecting belt longevity also. Should the tensioner fail, for example, the slack belt can simply come away from its pulleys.
As belt life is linked so directly to the health of those other components, it is wise to not replace the belt alone. For this reason, most timing belts are supplied as part of a kit that should include a tensioner and idler at the very least. However, you should consider taking the operation one step further, by renewing other items. Water pumps, for example, tend to require replacing at least every other belt change and this gives you the opportunity to refresh the coolant, renewing the cooling system’s anti-corrosive properties that diminish naturally with time.
Consider the condition of any parts that might fail and get tangled within the timing belt. Auxiliary, or balance shaft, belts can come off their pulleys and be dragged into the timing belt cover. Linked to this is a crankshaft pulley, many of which incorporate an internal damper (see Torsional vibration dampers – TVD).
As externally-mounted timing belts operate ‘dry’, the presence of oil, in particular, will degrade their structure, so attending to any coolant and oil leaks must be a priority. Obviously, no rotating belt should be allowed to abrade against anything and a part that has been fitted correctly should run freely – although missing bolts on the cover can cause the belt to rub through the plastic casing.
Belt life can also be shortened by incorrect fitting and careless handling. Never crimp or twist a belt, because you can break the internal reinforcing fibres. Never contaminate the belt with dirt or oil either – so keep your hands clean.
Apart from handling issues, getting the tension wrong is the most common mistake. Misaligning and overtightening the belt can be just as damaging as not tensioning it sufficiently. A belt that it too tight might promote a whining sound with the engine running; eventually, its teeth can strip. Overtensioning also places an additional strain on other components, such as the high-pressure diesel pump bearings.
Getting the tension right is not always easy and methods differ between engines. Some tensioners are spring-loaded. The one pictured here is not and requires a force of 30 newtons to be applied to the belt and its deflection checked as being 7.5mm at a specific point with the engine cold.